The result of polygon_90_set_data::get with argument vthreshold is incorrect

The code is as follows:
``` C++
#include "boost/polygon/gtl.hpp"
#include
#include

using namespace std;

int main(int argc, char** argv)
{
gtl::point_data point[] = {{53, 17}, {53, 68}, {33, 68}, {33, 17}};
gtl::polygon_90_data p90;
p90.set(point, point + 5);
gtl::rectangle_data r1 = {10, 20, 30, 40};
gtl::rectangle_data r2 = {10, 25, 35, 45};
gtl::rectangle_data r3 = {10, 55, 43, 62};
gtl::polygon_90_set_data ps1(gtl::VERTICAL), ps2(gtl::VERTICAL);
ps1.insert(r1);
ps1.insert(r3);
ps2.insert(r2);
ps2.insert(p90);

gtl::polygon_90_set_data ps3 = ps1 | ps2;
vector> p90s;
ps3.get(p90s, 8);
for (auto& p : p90s) {
for (auto& x : p) {
cout << "(" << x.x() << " " << x.y() << ")";
}
cout << endl;
}

return 0;
}
```
The result is:
```
(17 33)(25 33)(25 30)(20 30)(20 10)(45 10)(17 10)
(53 17)(33 17)(33 55)(10 55)(10 62)(33 62)(33 68)(53 68)
```
The input 17(point{53,17}) is the vertical coordinate, and the output 17(point{17,33}) is the horizontal coordinate.